ASTM D4628 for barium, calcium, magnesium and zinc in unused lubricating oils
ASTM D4628 standard test method for analysis of barium, calcium, magnesium, and zinc in unused lubricating oils by atomic absorption spectrometry
8. Preparation of Apparatus
8.1 Consult the manufacturer's instructions for the operation of the atomic absorption spectrophotometer. The present test method assumes that good operating procedures are followed. Design differences between spectrophotometers make it impractical to specify the required manipulations in detail here. (Warning - Proper operating procedures are required for safety as well as for reliability of results. An explosion can result from flame blow-back unless the correct burner head and operating sequence are used.)
8.2 For the barium determination, fit the barium hollow cathode lamp and set the monochromator at 553.6 nm. Make fine adjustments to the wavelength setting to give maximum output. Using the correct burner head for acetylene/nitrous oxide, set up the acetylene/nitrous oxide flame. On instruments where applicable, adjust the gain control to set this maximum at full scale, when aspirating standard (6) in 6.6.
8.3 Aspirate at about 2.5 to 3 mL/min a standard barium solution into the flame. Make adjustments to the height and angle of the burner and to the acetylene flow rate to give maximum absorption. Make sure that standard (6) in 6.6 still gives zero absorbance by making adjustments, if necessary.
9. Calibration (Barium)
9.1 Aspirate standard (1) in 6.6. With a minimum of scale expansion or burner rotation, obtain a reading of 0.5 more or less 0.1 on the absorbance meter or alternative read-out device.
9.2 Aspirate the standards of 6.6 sequentially into the flame and record the output (or note the meter deflections). Aspirate the solvent alone after each standard.
9.3 Determine the net absorbance of each standard. If the spectrophotometer output is linear in absorbance, the net absorbance is given by the difference between the absorbance for the standard or sample solution and the absorbance for the solvent alone. If the spectrophotometer output is proportional to transmission (that is, to light intensity) then the net absorbance is given by log10 d0/d1, where the deflections are d0 when solvent alone is aspirated and d1 when the standard or sample solution is aspirated.
9.4 Plot the net absorbance against the concentration (mg/50 mL suppressant solution) of barium in the standards to give a calibration curve.
NOTE 8 - The calibration curve may be automatically calculated by the instrument software and displayed by way of the instrument computer terminal, making actual plotting unnecessary.
9.5 Calibration must be carried out prior to each group of samples to be analyzed and after any change in instrumental conditions, as variation occurs in the instrument behavior. Readings may also vary over short times from such causes as buildup of deposits on the burner slot or in the nebulizer. Thus, a single standard should be aspirated from time to time during a series of samples to check whether the calibration has changed (a check after every fifth sample is recommended). The visual appearance of the flame also serves as a useful check to detect changes of condition.
9.6 Determine the slope and intercept for barium based on the calibration curve developed. The values will be used to determine barium concentrations of samples to be tested. Ensure that the regression coefficient is at least 0.99 for barium, otherwise the laboratory needs to re-calibrate for barium when this criteria is not satisfied.
